Synthesis of benzoxazine monomer with low curing temperature from renewable diphenolic acid, benzylamine and paraformaldehyde

Journal of Science and Technology 55 (1B) (2017) 63–69 SYNTHESIS OF BENZOXAZINE MONOMER WITH LOW CURING TEMPERATURE FROM RENEWABLE DIPHENOLIC ACID, BENZYLAMINE AND PARAFORMALDEHYDE Cao Xuan Viet 1, *, Tran Minh Hoan 1, Nguyen Thi Minh Nguyet 2 1Department of Polymer Materials, Faculty of Materials Technology, HCMUT–VNUHCM 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam 2Materials Technology Laboratory (MTLab), HCMUT–VNUHCM 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Viet Nam *Email: caoxuanviet@hcmut.edu.vn Received: 30 December 2016; Accepted for publication: 3 March 2017 ABSTRACT The aim of this study was to examine the use of renewable diphenolic acid (DPA) as starting materials together with benzylamine and paraformaldehyde for the synthesis of novel polybenzoxazine resin with low curing temperature. The monomer structure was confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. Differential scanning calorimetry (DSC) was also used to study crosslinking behavior of synthesized material. The benzoxazine monomer exhibited low exothermic peak with the onset around 162 °C, which is significantly lower than conventional benzoxazines derived from bisphenol A. The presence of carboxyl groups in monomer structure is responsible for the low polymerization temperature of this monomer. Keywords: diphenolic acid, renewable, bisphenol A, benzoxazine. 1. INTRODUCTION Currently, the use of petroleum based feedstock for the manufacture of polymeric materials leads to worldwide problems such as increasing CO2 concentrations in the atmosphere, global warming, and other environmental concerns about waste [1]. These problems have encouraged the scientific community to develop and commercialize new bio–based products that can reduce the dependence on fossil fuels and minimize the negative environmental effects while they can be less expensive and better performing. The use of renewable polymers is not just an academic curiosity. Renewables have the potential to provide a new and sustainable supply of basic chemical building blocks [2]. In recent years, polybenzoxazine (PBZ) has attracted a lot attention due to its unique advantages as compared with traditional phenolic resins, such as low dielectric constant, low water absorption, good thermal stability, high chemical resistance, and near–zero shrinkage upon curing. Despite having many advantages, PBZs have also exhibit some drawbacks. These Synthesis of benzoxazine monomer with low curing temperature from renewable diphenolic 64 drawbacks are the high temperature needed for the cure and the brittleness of the cured materials that can sometimes limit their potential applications [3]. To properly overcome these issues, several strategies such as: preparation of monomers with additional functionality, synthesis of novel polymeric precursors and blending with a high–performance polymer, and fiber, have been attempted [4]. Benzoxazine derived from bisphenol A (BPA) has been widely reported. The resulting thermoset resin with high structural integrity also possess very good properties such as superior heat resistance, electrical insulation and especially water resistance, compared with the cured resin from BPA type novolac and epoxy resin [3, 4]. However, studies indicate that BPA may cause several problems such as cardiovascular disease, type 2 diabetes and abnormalities in liver enzymes. Diphenolic acid (DPA) or 4,4’–bis(4–hydroxyphenyl)pentanoic acid is a chemical compound obtained by the condensation reaction of levulinic acid with phenol by using a catalyst. Levulinic acid is believed to be a cheap platform chemical and can be commercially produced from cellulose–rich biomass (especially from waster biomass) in large scale. DPA has similar chemical structure as BPA, lower price and it contains an extra functionality (carboxylic acid) that can be involved for the polymer synthesis. Therefore, it can be a good candidate to replace BPA for the synthesis of polybenzoxazine materials [5]. In this paper, we report the synthesis of benzoxazine from DPA, benzylamine and paraformaldehyde. The obtained benzoxazine monomer was also investigated by differential scanning calorimetry (DSC) to determine the curing temperature. 2. MATERIALS AND METHODS 2.1. Materials All reagents and solvents were used as received from commercial suppliers. Diphenolic acid (95 %), benzylamine (99 %) and sodium hydroxide (97% ) were purchased from Sigma– Aldrich. Paraformaldehyde (95 %) and chloroform (99 %) were obtained from Merck. Toluene (99 %) was provided by Prolabo. All reactions were carried out in oven–dried flask. 2.2. Synthesis of 4,4’–Bis–[6–(3–benzyl–3,4–dihydro–2H–1,3–benzoxazine)] Pentanoic Acid (DPA–b) Benzylamine (1.64 mL, 15 mmol) and paraformaldehyde (0.9 mg, 30 mmol) were dissolved in 20 mL toluene in two–necked round–bottom flask and stirred until it became homogeneous at 70 °C for 1 h. Then, diphenolic acid (2.15 g, 7.5 mmol) was loaded into the flask. The reaction mixture was heated at 100 °C for 6 h. The resulting light orange solution was filtered and concentrated under vacuum to obtain a syrup that was subsequently dried under high vacuum giving a yellowish solid with a yield of 79 % of DPA–b. 1H NMR (CDCl3/TMS, δ ppm): 7.34–6.70 (16H, Ar–H), 4.83 (4H, s, O–CH2–N), 3.91 (4H, s, Ar–CH2–N), 3.90 (4H, Ar–CH2–N) 2.35 (2H, t), 2.13 (2H, t), 1.52 (3H, s); 13C NMR (CDCl3, δ ppm): 179.0 (s), 152.2(s), 141.0.5 (s), 138.0.1 (s), 129.1 (d), 128.5 (d), 127.5 (d), 126.8 (d), 119.5 (s), 116.0 (d), 81.9 (d), 55.6 (t), 50.3 (t), 44.7 (s), 36.7 (t), 30.5 (t), 27.8 (q); FTIR: υcm–1) 1708 (C=O st). Cao Xuan Viet, Tran Minh Hoan, Nguyen Thi Minh Nguyet 65 2.3. Polymerization of DPA–b Monomer DPA–b (500 mg, 9.11×10–4 mol) was dissolved in 30 mL of anhydrous THF and then was dropped on the PET film. The monomer sample was heated at 100 °C for 1 h and continued heating for another 2h at 200 °C. 2.4. Measurements Fourier transform infrared (FTIR) spectra of the samples were recorded on a Bruker Tensor37 spectrophotometer with a resolution from 4000 – 400 cm-1 in the absorbance and transmittance modes. The test was done at Institute of Chemical Technology, Vietnam Academy of Science and Technology (VAST), Ho Chi Minh City. 1H (500 MHz) and 13C (125.8 MHz) nuclear magnetic resonance (NMR) spectra were obtained using a Bruker Avance AM500 FT-NMR spectrometer with Fourier transform and CDCl3 as solvent. The chemical shift are given relative to tetra methyl silane (TMS). The NMR measurements and analysis were performed at Institute of Chemistry, Vietnam Academy of Science and Technology (VAST), Hanoi. Differential scanning calorimetric (DSC) studies were carried out on a Mettler Toledo thermal analyzer using N2 as a purge gas, heated from room temperature to 300 °C at scanning rate of 10 °C/min. DSC measurements were carried out at Central Laboratory for Analysis, Ho Chi Minh City University of Science. 3. RESULTS AND DISCUSSION 3.1. Synthesis of diphenolic acid based benzoxazine monomer A diphenolic–based benzoxazine DPA–b was prepared by reaction of diphenolic with benzylamine and formaldehyde in molar ratio of 1:2:4 as can be seen in Figure 1. Figure 1. Synthesis of DPA–b. Figure 2 shows 1H NMR and 13C NMR spectra of the novel benzoxazine monomer. It can be seen that the nuclear magnetic resonance spectroscopy (1H NMR) spectrum of the DPA–b exhibits not only the specific signals of the benzoxazine ring, but also chemical shifts that belong to the alkyl chain and the aromatic signals. Notably, while the two signals at 4.83 and 3.91 ppm correspond to –CH2 protons of benzoxazine ring, methyl protons of the pentanoic acid appears at 1.52 ppm (singlet, 3H). Alkyl protons of the propionic acid moiety resonate at 2.13 ppm (triplet, C–CH2, 2H) and 2.35 ppm (triplet, –CH2–COOH, 2H). Alkyl protons of the benzylamine (–CH2) correspond at 3.90 ppm. The COOH proton is not observed when CDCl3 is used as solvent [6]. Sy 66 tw –N nthesis of be The corre o singlets at –CH2–Ph of nzoxazine m sponding 13C 81.9 ppm an the oxazine Figure 2. onomer wit NMR spec d 55.6 ppm ring, respec 1H NMR (a), h low curing trum also su are typical tively [6]. 13C NMR (b) temperature pports the st of the carbo spectra of D from renew ructure of th n resonances PA–b monom able diphen ese compou of –N–CH2 er. olic nds. The –O– and (Fi pe str the 3.2 20 hig aro low the (T str he cat bri wi rea ox Moreover gure 3). The aks at about etching mod C–N–C sym . Polymeriz Figure 4 d 0 °C for 2 h h curing tem und 200–26 compared rmogram re able 1). The ucture of mo ated to high alyst H+ atta dge structur Thermal c th Mannich ctions betw azine ring. T , the expect presence of 1231 and es, respectiv metric stret ation of DP isplays the ). It is well perature. T 0 °C. Howe to those o vealed a br low polym nomer char enough tem ck the anion e [7]. uring of DP bridges. Su een carboxy he possible C ed structure cyclic ether 1023 cm–1 d ely. The ab ching and as Figure 3. FTI A–b monom DSC thermo known that he ring ope ver, the pol f the typica oad exother erization tem acteristic, th peratures, c O– of oxaz A–b was inv bsequently, lic and phen chemical str ao Xuan Vi of the mon of benzoxa ue to the C sorbance pe ymmetric st R spectrum o er grams of th one main s ning temper ymerization l benzoxazi m with an o perature o at is, to the p arboxylic gr ine ring and olved in ox additional c olic hydrox uctures of th et, Tran Min omer was fu zine structur –O–C symm aks at 1169, retching mo f DPA–b mo e DPA–b m hortcoming ature of bis temperature nes derived nset at 162 f this monom resence of c oup will fo the polyme azine ring op rosslinking yl groups fo e polymer il h Hoan, Ngu rther exami e is confirm etric stretch 1119, 826 c des, respecti nomer. onomer and in benzoxaz phenol A ba observed f from bisph °C and a m er can dire arboxylic g rm intermed rization form ening, givin takes place rmed durin lustrated in F yen Thi Min ned by the ed by the ab ing and asy m–1 are attr vely [6, 7]. its polymer ine chemist sed benzox or DPA–b w enol A [8, aximum a ctly be attr roup (COOH iate ion H+ ed thought g phenolic s due to este g the openin igure 5. h Nguyet 67 IR result sorbance mmetric ibuted to (cured at ry is the azines is as quite 9]. The t 191 °C ibuted to ). When . Cation Mannich tructures rification g of the Synthesis of benzoxazine monomer with low curing temperature from renewable diphenolic 68 Figure 4. DSC thermograms of DPA–b monomer and DPA–b polymer. Table 1. DSC data of the DPA–b and its polymer. Sample Peak 1 Peak 2 Tonset (°C) Tmax (°C) ∆H (J/g) Tonset (°C) Tmax (°C) ∆H (J/g) DPA–b 162.5 191.2 53.2 236.5 238.3 2.8 DPA–b polymer 129.1 179.8 – 248.9 273.6 11.8 Figure 5. Possible chemical structures involved in the crosslinking process. Peak endothermic at 238 °C can be explained that when heated over 200 °C, the sample starts to degrade and this decomposition practically coincides with the beginning of the CO2 evolving through the decarboxylation reaction [6]: After the decarboxylation reaction completed and carboxylic group was over, the ring opening of DPA–b continued at 243 °C as normal. However, for the polymer sample cured at 200 °C, there is only one endothermic peak appeared at 218 °C which is attributed to the Cao Xuan Viet, Tran Minh Hoan, Nguyen Thi Minh Nguyet 69 decarboxylation reaction. Apparently, this result suggests the completion of the polymerization of DPA–b after the heat treatment. Zúñiga et al. [5] studied the curing behavior of polybenzoxazine derived from diphenolic acid, aniline and paraformaldehyde. Interestingly, the material did not undergo the decarboxylation when the sample was slowly heated below 190 °C for several hours with the presence of external pressure. The possible explanation for this phenomenon may be due to the carboxylic group was mainly functioned as a catalyst for the polymerization. 4. CONCLUSIONS We have successfully synthesized a novel benzoxazine monomer from renewable diphenolic acid, benzylamine and paraformaldehyde by a solvent method. The monomer structure were well confirmed by the FTIR, 1H NMR and 13C NMR. 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